Hahaha XD
Don't worry, that confused the hell out of me when I first joined, It's one thing I really don't like about this site. To reply to a comment, you just have to go post on the commenter's wall on their account. And it's get's really confusing the more people talk to you, because there's no way to keep track of what you've replied to or not, or what they said in the first place (because by the time I go to their page to leave a comment in response to theirs, I totally forget what we were talking about OTL)

Yeah I give up when it comes to saying 'thanks for the watch or favorite' because I lose track of everything. I could focus on one conversation at a time, but that's about it. I feel bad for the FA famous lol

Probably for the best ;)

But that's only on wall posts, you can reply like normal to journals and artwork.